After Input Call Method Again Java
The event loop
JavaScript has a runtime model based on an upshot loop, which is responsible for executing the lawmaking, collecting and processing events, and executing queued sub-tasks. This model is quite unlike from models in other languages like C and Java.
Runtime concepts
The following sections explicate a theoretical model. Modern JavaScript engines implement and heavily optimize the described semantics.
Visual representation
Stack
Function calls form a stack of frames.
office foo ( b ) { let a = 10 return a + b + 11 } function bar ( x ) { let y = three return foo (x * y) } const baz = bar ( seven ) // assigns 42 to baz Guild of operations:
- When calling
bar, a start frame is created containing references tobar's arguments and local variables. - When
barcallsfoo, a second frame is created and pushed on superlative of the first one, containing references tofoo's arguments and local variables. - When
fooreturns, the top frame element is popped out of the stack (leaving onlybar'southward call frame). - When
barreturns, the stack is empty.
Note that the arguments and local variables may keep to exist, as they are stored outside the stack — and so they can be accessed by any nested functions long after their outer function has returned.
Heap
Objects are allocated in a heap which is but a name to denote a large (mostly unstructured) region of retention.
Queue
A JavaScript runtime uses a message queue, which is a list of messages to exist candy. Each message has an associated part that gets chosen to handle the message.
At some signal during the event loop, the runtime starts treatment the messages on the queue, starting with the oldest one. To do so, the message is removed from the queue and its corresponding function is called with the message as an input parameter. Equally always, calling a function creates a new stack frame for that function'southward use.
The processing of functions continues until the stack is once again empty. So, the event loop volition process the next bulletin in the queue (if at that place is one).
Event loop
The result loop got its proper name because of how it's normally implemented, which usually resembles:
while (queue. waitForMessage ( ) ) { queue. processNextMessage ( ) } queue.waitForMessage() waits synchronously for a message to arrive (if one is not already bachelor and waiting to be handled).
"Run-to-completion"
Each message is processed completely earlier any other message is processed.
This offers some nice properties when reasoning about your program, including the fact that whenever a office runs, it cannot be preempted and will run entirely before any other code runs (and can change data the function manipulates). This differs from C, for case, where if a role runs in a thread, it may be stopped at any point by the runtime system to run some other lawmaking in another thread.
A downside of this model is that if a message takes too long to consummate, the web application is unable to process user interactions like click or scroll. The browser mitigates this with the "a script is taking too long to run" dialog. A good practice to follow is to brand bulletin processing brusk and if possible cutting downward ane bulletin into several messages.
Adding letters
In spider web browsers, messages are added someday an outcome occurs and there is an event listener attached to it. If at that place is no listener, the event is lost. Then a click on an element with a click event handler will add a message—likewise with any other consequence.
The function setTimeout is called with 2 arguments: a message to add to the queue, and a time value (optional; defaults to 0). The time value represents the (minimum) delay after which the message will be pushed into the queue. If there is no other message in the queue, and the stack is empty, the message is processed right after the filibuster. However, if there are messages, the setTimeout message volition have to await for other messages to be processed. For this reason, the 2d argument indicates a minimum time—not a guaranteed time.
Here is an example that demonstrates this concept (setTimeout does non run immediately after its timer expires):
const seconds = new Appointment ( ) . getSeconds ( ) ; setTimeout ( function ( ) { // prints out "2", pregnant that the callback is not called immediately afterward 500 milliseconds. console. log ( ` Ran later ${ new Date ( ) . getSeconds ( ) - seconds} seconds ` ) ; } , 500 ) while ( true ) { if ( new Appointment ( ) . getSeconds ( ) - seconds >= 2 ) { panel. log ( "Practiced, looped for 2 seconds" ) suspension ; } } Zero delays
Zip delay doesn't mean the call back volition fire-off after nothing milliseconds. Calling setTimeout with a delay of 0 (cypher) milliseconds doesn't execute the callback function after the given interval.
The execution depends on the number of waiting tasks in the queue. In the example below, the bulletin "this is just a message" will be written to the console earlier the message in the callback gets candy, because the delay is the minimum time required for the runtime to process the asking (not a guaranteed time).
The setTimeout needs to wait for all the lawmaking for queued messages to complete even though yous specified a item time limit for your setTimeout.
( function ( ) { panel. log ( 'this is the starting time' ) ; setTimeout ( function cb ( ) { console. log ( 'Callback 1: this is a msg from call up' ) ; } ) ; // has a default time value of 0 panel. log ( 'this is but a message' ) ; setTimeout ( function cb1 ( ) { console. log ( 'Callback 2: this is a msg from call up' ) ; } , 0 ) ; console. log ( 'this is the end' ) ; } ) ( ) ; // "this is the first" // "this is just a message" // "this is the stop" // "Callback one: this is a msg from call back" // "Callback 2: this is a msg from telephone call back" Several runtimes communicating together
A spider web worker or a cantankerous-origin iframe has its own stack, heap, and message queue. Two singled-out runtimes tin can just communicate through sending messages via the postMessage method. This method adds a bulletin to the other runtime if the latter listens to message events.
Never blocking
A very interesting property of the event loop model is that JavaScript, unlike a lot of other languages, never blocks. Handling I/O is typically performed via events and callbacks, so when the application is waiting for an IndexedDB query to render or an XHR request to return, it can however procedure other things like user input.
Legacy exceptions exist like alert or synchronous XHR, simply it is considered good practice to avoid them. Beware: exceptions to the exception practice exist (but are normally implementation bugs, rather than anything else).
Run into also
- Event loops in the HTML standard
- Node.js Event Loop
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Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop
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